This invention relates generally to induction heating apparatus, and more particularly to induction heating apparatus which allows the heater unit to be a hand-held portable device.
Induction heating is a relatively well known expedient for producing heat in localized areas on metal objects by the use of RF power which develops a varying electromagnetic field within the metal to be heated. The internal energy losses within the material cause it to heat up in a relatively short period of time.
However, one of the primary disadvantages of induction heating units heretofore utilized is that the RF power which is generated by the oscillator circuit has to be delivered over a transmission line to the induction heating element. By so transmitting the RF power, much of the energy generated within the system is lost and the general efficiency of the system is greatly reduced. This is not a significant disadvantage when utilizing induction heating units which have large receptacles wherein the components to be heated are placed or which have a chamber or container about which the heating elements are positioned. However, when small localized areas of large surfaces are to be heated the induction heating unit must take a different form and the problem of transmission line losses becomes significant.
One illustrated use of the induction heating apparatus of this invention is in connection with the automotive industry. Heretofore, automobiles have molding strips fastened by drilling holes through the side wall of the doors and fenders of the cars. To eliminate the drilling of holes, molding clips can be adhesively fastened to the surface of the automobile and the chrome or stainless steel decorative strip is then fastened to the glued on clip. By thus eliminating the holes through the sides of the automobiles, the amount of rust and corrosion which would otherwise occur within the interior of the car body is substantially reduced.
Because of the high rate of mass production of automobiles, it is necessary to have a clip which can be adhesively fastened to the side of an automobile quickly, within a matter of seconds, and which will not damage the already finished surface. This can be accomplished by utilizing thermosetting adhesive materials on the back side of the clip and heating the adhesive material, either directly or indirectly, to a temperature sufficiently high to cause it to thermally set. However, the temperature must not be so high as to damage the painted surface of the automobile or cause it to loosen from the parent metal. A securely bonded painted surface of an automobile can then receive a second bonded element which will form a strong connection between the second element and the base metal under the paint.
When utilizing thermosetting adhesive materials on molding clips as mentioned above it was found necessary to control the amount of heat, both its temperature and the time of application, within precise limits. For example, an improved form of thermosetting adhesive can be made plastic and become an adhesive at an elevated temperature of about 250.degree. to 300.degree. F. This must be done in a matter of about two to three seconds so that the elevated temperature will not damage the painted surface to which the adhesive is applied. Because of the short time duration of the applied heat, it is advantageous to use a heating apparatus which can be controlled either electrically or electronically to the precise limits necessary. Also, the heating unit must be small and easy to handle, and energy losses over the transmission lines between the power supply and the heating unit must be minimized so that the overall efficiency of the system is high and economically feasible.